Microencapsulation technology allows to encapsulate a compound inside a tiny sphere, known as a microsphere or a microcapsule, having an average diameter as small as 1 millimeters to several micrometers. Many different active materials like drugs, enzymes, vitamins, pesticides, flavours and catalysts have been successfully encapsulated inside microcapsules made from a variety of polymeric and non-polymeric materials including poly(ethylene glycol)s, poly(methacrylate)s, poly(styrene)s, cellulose, poly(lactide)s, poly(lactide-co-glycolide)s, gelatin and acacia, etc. These microcapsules release their contents (active material) when needed thanks to different release mechanisms, which depend on the end use of the encapsulated products. This technology has been used in several fields including pharmaceutical, agriculture, food, printing, cosmetic and textile.
In particular, aminoplast core/shell microcapsules are suitable for the encapsulation of active materials for the cosmetic, textile, and agrochemicals applications.
Aminoplast microcapsules represent a widely used and industrially relevant approach in the field of microencapsulation. There are established processes of forming aminoplast microcapsules that are well documented in the prior art. Typically, in a first step, an oil-in-water emulsion is formed. This emulsion consists of active material-containing oil droplets dispersed in an aqueous continuous phase. Thereafter, shell-forming monomers or pre-condensates contained in the emulsion allow the formation of an encapsulating polymeric shell around the active material-containing droplets and therefore lead to the formation of core-shell microcapsules.
Reagents and reaction conditions are selected to ensure efficient migration of the monomers or pre-condensates to the oil-water interface so that the polymeric shells can form rapidly around the oil droplets, thereby retaining all, or substantially all, of the active material within the cores and preventing leakage of encapsulated active material from the microcapsule cores. If the shell-forming materials do not migrate to the oil-water interface quickly or in sufficient amounts, it may be impossible to form microcapsules. In that case, if microcapsules are formed, they may be characterized by poor active material retention and may be prone to agglomeration.
Polymers, acting as protective colloids stabilizer, are employed to stabilize the oil-water interface during the microcapsule formation. The polymeric stabilizer functions in several ways: it ensures that stable oil-in-water emulsions are formed; it facilitates migration of monomers and pre-condensates to the oil-water interface; and it provides a template around which the monomers or pre-condensates can react to form the encapsulating polymeric shells.
Polymeric stabilizers employed in the preparation of aminoplast microcapsules are anionic or non-ionic polymers, see for example U.S. Pat. No. 8,119,587. Particularly effective polymeric stabilizers are acrylic acid-based copolymers bearing sulphonate groups. Examples of commercially available copolymers include LUPASOL® (ex BASF), such as LUPASOL PA 140, or LUPASOL VFR. These commercial polymers are exemplary polymeric stabilizers, which are employed in the preparation of commercial aminoplast microcapsule compositions.
The aminoplast microcapsules prepared by the process described above are typically collected in the form of a slurry comprising a plurality of microcapsules suspended in a suitable suspending medium. The microcapsule slurry may then be used directly in applications, or further processed. For example, it is conventional to post-coat aminoplast microcapsules with a cationic water-soluble polymer in order to provide them with a net positive charge. This coating acts as a deposition aid and increases the substantivity of the microcapsules when deposited on certain substrates. However, post-coating requires an extra step, which increases the cost of the manufacturing process. Indeed, large amounts of cationic polymer are necessary to first neutralize the negatively charged microcapsules before these microparticles can finally have a net positive charge.
As a result, there is a need to develop microcapsules with an increase of stability and a better control of the thickness. Moreover there is a need to improve the preparation of microcapsules without using conventional post-coating techniques.